Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase, shift translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, the color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity. The power Doppler mode also displays sample volumes laid over a B-mode image, but the displayed sample volumes are color-coded to represent the power or energy of the signals reflected from each sample volume.
In standard color flow processing, a high pass filter known as a wall filter is applied to the data before a color flow estimate is made. The purpose of this filter is to remove signal components produced by tissue surrounding the blood flow of interest. If these signal components are not removed, the resulting velocity or power estimate will be a combination of the velocity or power of the signal returned from the blood flow and the velocity or power of the signal returned from the surrounding tissue. The backscatter component from tissue is many times larger than that from blood, so the parameter estimate will most likely be more representative of the tissue, rather than the blood flow. In order to get the flow velocity or power, the tissue signal must be filtered out.
In the color flow mode of a conventional ultrasound imaging system, an ultrasound transducer array is activated to transmit a series of multi-cycle (typically 4-8 cycles) tone bursts which are focused at the same transmit focal position with the same transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. A series of transmit firings focused at the same transmit focal position are referred to as a "packet". Each transmit beam propagates through the object being scanned and is reflected by ultrasound scatterers such as blood cells. The returned signals are detected by the elements of the transducer array and then formed into a receive beam by a beamformer.
For example, the traditional color firing sequence is a series of firings (e.g., tone bursts) along the same position, which firings produce the respective receive signals: EQU F.sub.1 F.sub.2 F.sub.3 F.sub.4 . . . F.sub.M
where F.sub.i is the receive signal for the i-th firing and M is the number of firings in a packet. These receive signals are loaded into a corner turner memory, and a high pass filter (wall filter) is applied to each down range position across firings, i.e., in "slow time". In the simplest case of a (1, -1) wall filter, each range point will be filtered to produce the respective difference signals: EQU (F.sub.1 -F.sub.2)(F.sub.2 -F.sub.3)(F.sub.3 -F.sub.4) . . . (F.sub.M-1 -F.sub.M)
and these differences are input to a color flow power estimator.
Power Doppler imaging maps the power or energy in blood flow over a two-dimensional image. The transfer function of the resultant displayed power Doppler image is the product of the transfer function of the underlying compression curve and a mapping function. State-of-the-art ultrasound systems typically give the user a series of mapping functions from which to choose. These mapping functions provide for an arbitrary selection of displayed colors, but do not allow the user or the system to optimize the image information based either on user settings or imaging data itself.